Liquified Petroleum Gas (colloquially referred to as Propane) may be compressed to a transportable liquid when used as a fuel within engine systems. However, to ensure that the propane remains a liquid during engine operations, additional measures may also be included that increase the complexity and cost therein. For example, to inject a liquid fuel via port fuel injection, the fuel rail may be cooled by returning the engine-heated fuel within the fuel rail to the fuel storage tank. However, such recirculation may result in storage tank heating as the returning hot liquid continually mixes with the cooler liquid remaining in the storage tank, which therefore limits the applicability of the liquid injection propane fuel system when operated in hot environments. Alternatively, to inject a fuel via direct fuel injection, intricate devices may be included within the fuel system whose function is to cool and pressurize the propane fuel while keeping it in the liquid phase and below the critical point.
Another approach may mix two fuels having different chemical compositions to enhance the thermal response of the fuel mixture. For example, U.S. Pat. No. 7,861,696 discloses a multi-fuel supply and co-injection system that mixes various combinations of fuels (both liquid and gaseous) together under the control of a microprocessor in a manner that enhances the utilization of the thermal content of the fuels based on the combustion efficiency and power output within the engine. However, the disclosed system also includes components, such as a fuel circulation pump, an accumulator, and a positive displacement pump for increasing the pressure of the gaseous propane in order to keep it a liquid within the engine system. Such components may degrade during operation, and thus temperature and/or pressure control of the fuel may correspondingly degrade, resulting in potential fueling errors.
The inventors have recognized the above issues, and others, with such approaches and herein disclose various embodiments, several of which include a method for adjusting a fuel composition to increase the liquid fuel heat tolerance of a first fuel (e.g., herein LPG) by introducing a second fuel (e.g., herein gasoline) to the first fuel in response to elevated fuel temperatures. For example, mixing the two fuels allows the fuel mixture to remain in the liquid phase and thereby prevents the first fuel from becoming a supercritical fluid during engine operation, which presents problems for engine pump and injector performance. Herein, one approach is described wherein the method includes directly injecting liquid propane to the engine via a fuel rail when a fuel temperature falls below a threshold; and in response to a fuel temperature greater than a threshold, directly injecting a liquid mixture of propane and gasoline via the fuel rail. The method further allows for the composition of the liquid mixture to be controlled by metering the amount of gasoline added to the liquid propane to ensure the mixture remains a liquid during engine operations. In this way, the technical result is achieved that the operating range of the first liquid fuel may be extended which may allow for fuel cooling to be reduced (or substantially eliminated in some instances) and the fuel pressurization to be reduced.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings. It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.